Dexter GPS Timestamp


#1

Hello,

I wanted to know a little bit about the timestamp provided by the shield. If there is no GPS signal, does the timestamp get updated by some sort of IC? Will the time update regardless of wether the position had ever been recieved? (assumming there was an initial connection to sync the clock at some point in its life)

Also, what is the purpose of the battery? I read in the documentation that it stores the almanac in SPRAM. Is it also used for some sort of IC?

Thanks


#2

Yes, that’s right. If the GPS signal is lost, the internal clock keeps increment. When the GPS signal is regained, the clock is updated (the internal clock is pretty accurate though). if you remove the battery and the memory is dumped, you’ll see the clock goes right back to a few minutes before midnight.

The battery keeps the SRAM alive and some of the circuitry to keep the clock alive. Basically, it helps keep all the memory updated when VCC is disconnected.

Thank you for the question!


#3

Than you for the reply.

This is a great feature of this gps chip. I dont know if I recall reading that specifically in any documentation.

Couple more questions:

  1. As far as detecting failures in the gps. If the time is kept on the RTC, and a GPS message is reported invalid. Can I trust the timestamp to be valid still? What does this tell me about the timestamp in an invalid message?

  2. Your comment regarding the battery… wasnt sure if you were assuming i put it back in or not. What happens if the battery dies? Will the shield continue to work? Im guessing it does, it will just take longer to align. How can I detect this failure mode? Im guessing I will have enough time to check for that “two min” before midnight as a failure mode during this extended time for alignment.

3)Just curious… Any idea why the clock resets back to “two min” before midnight, instead of midnight?


#4

Thanks!

1). I think that depends on how accurate you want the time to be. The RTC works quite well. You can only poll it at 1 time per second. You’ll probably not lose any accuracy if you lose site of satellites.

2). If the battery dies, the shield won’t work. You’ll need to replace it. If the battery dies, you won’t get any response from the GPS.

3). No idea, it’s curious right? No idea, other than I think it’s in there for developers: midnight is 00, so this is a more proof positive way of showing the time is reset and there’s not a problem with the software. Just a guess; it could be a mistake too!